, coinciding in part with the exterior sides of the abutments.
There is a horizontal thrust of the earth on the medial plane,
, acting at right angles to the plane of the paper, which tends to distribute the weight of the central portion partly toward the sides; but, ignoring this, it is seen that, if the earth everywhere settles uniformly, the maximum pressure per square unit at the top of the culvert is
, and the total vertical pressure on the culvert is the weight of the earth vertically above it.
If, however, the earth outside the abutment walls settles more than the walls (a case which may occur), then part of its weight, and that of the earth vertically above it, will be transferred, through friction and cohesion, along the planes,
and
